Information can be stored on and retrieved from sectors allocated on rotating magnetic or optical mass-storage devices such as the disks of a hard disk drive. The surface of given disk is typically divided into a number of concentric tracks, each track being divided into a series of arcuate sectors which store data. In order to read or write data to or from a sector on the disk, the head of the disk unit must first be properly positioned relative to the sector. This involves a three-stage process. First, the head is moved radially with respect to the disk until it is aligned with the track containing the requested sector, then the disk must rotate to a position in which the sector in question is about to move past the head, and then the data must be read or written. These three times are commonly referred to as the seek time, the latency time and the data transfer time.
In some conventional drives, a controller for two or more disk drives specifies the sector which is to be accessed on a particular drive. That drive then moves its head until the head is radially aligned with the track containing that sector, and then at the end of this seek time the controller is notified that the track has been located. Then, during the latency time, the head reads successive sectors from a starting point of the track, the controller examines them until it determines that the sector of interest has been found, and then it accepts the data from that sector. Statistically, therefore, the average latency time is approximately the interval required for the disk to rotate 180.degree.. Obviously, having the controller dedicated to a single disk drive throughout its latency time is inefficient.
The present inventor previously developed a technique for reducing this inefficiency, in particular by providing a circuit which can monitor the angular position of the disk in each of several disk drives during the latency time of each, and which provides notification to the controller shortly before the sector of interest in each drive comes into alignment with its respective head, so that the controller is not dedicated to each disk drive throughout its latency time but instead dedicates itself to any given disk drive only for an interval which is approximately the time required for two sectors on that disk to move past the head. This arrangement is disclosed in the inventor's U.S. Pat. No. 4,647,991, which issued on Mar. 3, 1987. However, the arrangement disclosed in this prior patent is intended specifically for use with conventional disks of the type in which each track has the same number of sectors.
More specifically, due to the circular shape of the traditional disk, information is typically disproportionately distributed on each track, because the outer tracks are divided into same number of sectors as the inner tracks. One way to remedy this disproportionate distribution of information is the known technique of zone-bit-recording. In zone-bit-recording, several concentric zones are defined which each include one or more tracks, and each zone is proportionately divided into a different number of sectors, depending on the amount of space available in each zone. Through the use of zone-bit-recording, a substantially larger amount of information can be recorded on a given disk than is the case where the same number of sectors are provided in every track on the disk. However, conventional disk drives which use zone-bit-recording techniques still require that the associated controller remain dedicated to the particular drive during the latency time and examine data from every sector read by the head during the latency time.
It is therefore an object of the present invention to provide an arrangement for a zone-bit-recording drive which avoids the need to have the controller dedicated to the drive throughout the latency time of the drive.
It is a further object of the present invention to provide such an arrangement which is compatible with existing standard disk interfaces such as the Enhanced Small Disk Interface (ESDI).
It is a further object of the invention to provide such an arrangement which involves a minimal amount of additional hardware, and thus minimal expense and no significant decrease in reliability.
The objects and purposes of the invention, including those set forth above, are met according to one form of the present invention by a method of sensing the position of a head of a zone-bit-recording disk drive relative to a rotating platter thereof which has a surface divided into at least two zones each having a respective different number of angularly spaced sectors therein, by carrying out the steps of: maintaining a count of the number of sectors which pass the head after the head passes a predetermined angular orientation, ignoring the count during a time interval when the head moves out of a first of the zones toward a second of the zones which contains the specified sector and until the head has entered the second zone and the platter thereafter passes a predetermined angular orientation, and thereafter determining when the count corresponds to the specified sector to be accessed.
According to another form of the present invention, an apparatus includes a zone-bit-recording disk drive having a rotating platter and a head movable relative to a surface of the platter which is divided into at least two zones each having a respective different number of angularly spaced sectors therein, a first arrangement for maintaining a count of the number of sectors which pass the head after the platter passes a predetermined angular orientation, a second arrangement responsive to the count of the first arrangement for determining when the count corresponds to a specified sector to be accessed, and a third arrangement for disabling the second arrangement during an interval when the head moves out of a first of the zones toward a second of the zones which contains the specified sector and until the head has entered the second zone and the platter has thereafter passed a predetermined angular orientation.